Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface showing a first image in a sequence; and,

FIG. 2 is a front view showing a second image in the sequence.

The longer-incremented broken lines show a display screen portion to illustrate environment and form no part of the claimed design. The dash-dot-dash broken lines of certain features define boundaries for those features and form no part of the claimed design. The finer-incremented broken lines in the remainder of the graphical user interface form no part of the claimed design.

The grayscale in the drawings indicates a contrast of appearance.

The subject matter in this patent includes a process or period in which an image changes into another image. The process or period in which an image transitions to another image forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 